Studies On The Structure And Hydrogen Storage Properties Of Zirconium-,Tin-and Magnesium-Tin-Aluminium Hydrides

Energy crisis and environmental pollution are two hot issues in today’s society.In order to solve such problems efficiently,the best feasible way is to find renewable energy and clean energy to replace traditional fossil fuels.Hydrogen energy is considered the ideal alternative because it is widely available,lightweight,clean and renewable.However,hydrogen storage has become a major bottleneck for the further development of hydrogen energy.To find a safe,efficient and economical method of hydrogen storage method is the key element for the development of hydrogen energy.There are certain security risks for high pressure gaseous hydrogen storage and low temperature liquid hydrogen storage.Therefore,the research and development of solid hydrogen storage materials has made great progress in the past few decades.Among various types of solid hydrogen storage materials,aluminum hydride is a kind of material with high hydrogen storage capacity.It stands out because of its advantages such as high hydrogen storage capacity,rich chemical composition,reversible storage possibility and no other volatile gases except hydrogen.But in aluminum hydride,Al-H bond has strong covalent interaction.Previous researchers generally carried out researches on light metal aluminum hydrides,but the strong ionic bond between light metal cations and[Al H₄]^-leads high dehydrogenation temperature of light metal aluminum hydrides.The higher the electronegativity of metal cations in aluminum hydrides,the weaker the covalent interactions of the Al-H bonds,the lower the dehydrogenation temperature of the materials,and more conducive to the desorption of hydrogen.In this paper,we use the global optimization algorithm to determine the crystal structures of Zr（Al H₄）₄,Sn（Al H₄）₄ and Mg Sn（Al H₄）₆,and investigate their stability,electronic properties and bonding properties by density functional theory.In the most stable structure of crystal Sn（Al H₄）₄ and Mg Sn（Al H₄）₆,there are triple-ring structure units composed of Al H₅ or Al H₆.The presence of free H₂ molecules in all three of the most stable crystal structures may be due to the failure to reach saturation in accordance with stoichiometric ratios during hydrogenation of the compounds.The enthalpy of formation of Zr（Al H₄）₄ and Mg Sn（Al H₄）₆ reaches 1.0 e V,while the enthalpy of formation of Sn（Al H₄）₄ is slightly lower.Metal atoms（Al,Sn,Zr）and H atoms form polarized covalent bonds.In Sn（Al H₄）₄,the covalent interaction of the Sn-H bond is slightly stronger than the covalent contribution of the Al-H bond,but the charge values on the Sn atom are much smaller.The thermodynamic instability may be due to the weak interlayer binding of Sn（Al H₄）₄ structure.In Zr（Al H₄）₄,both the covalent interaction and ionic interaction of Zr-H bonds are significantly weaker,and the charge values on Zr atoms are smaller than those on Al atoms.Compound Zr（Al H₄）₄ is stable at room temperature and may have good reversible properties.In Mg Sn（Al H₄）₆ the charge values on both Mg and Sn atoms are smaller than those on Al atoms.The Sn-H_b bond and Mg-H_b bond have significantly weaker covalent interactions than Al-H_b bond,while Mg-H_b bond has stronger ionic properties.It can be seen that the introduction of metal Mg atom weakens the covalent interaction of Sn-H_b bond significantly.Compounds Zr（Al H₄）₄ and Mg Sn（Al H₄）₆have hydrogen storage capacities of 7.49 wt%and 7.43 wt%,respectively,and they are promising hydrogen storage materials working at mild temperatures.